It has always been recognized that in a world wholly governed by Christian principles war would be ruled out; nevertheless, since Christians are members of a secular society in which the use of force is necessary to maintain the authority of law, it has been widely, though not universally, held that war and Christian participation in it are on occasion morally justified and even praiseworthy. In early times, when forms of civil government were pagan, some ecclesiastical enactments seemed to forbid Christians from taking part in military service, but there were Christians in the army from the 2nd cent. onwards. From the time of Constantine, Christians were less troubled by scruples about participation in war. The Crusades are the classic example of warfare undertaken for supposedly religious ends. Medieval moral theologians came to distinguish between wars in which a Christian could or could not legitimately take part. St Thomas Aquinas lays down three conditions for a ‘just war’: that it must be on the authority of the sovereign; that the cause must be just; and that the belligerents should have a rightful intention. F. de Vitoria (d. 1546) adds that the war must be waged by ‘proper means’.
In modern times ‘Absolute Pacifism’, that is the doctrine that warfare is in all circumstances forbidden by the Gospel, has been upheld by various groups of people, including Church leaders. It has also been argued that participation in a ‘just war’ is no longer possible, since the means (weapons of mass destruction and especially nuclear weapons) are never ‘proper’. The main stream of Christian thought, however, has not supported modern pacifist movements, on the ground that there are even worse evils than physical destruction.